This invention relates to coating materials, such as thermal insulating coatings for components exposed to high temperatures. More particularly, this invention is directed to a method of reducing the thermal conductivity of a thermal barrier coating (TBC) formed of yttria-stabilized zirconia.
Higher operating temperatures for gas turbine engines are continuously sought in order to increase their efficiency. However, as operating temperatures increase, the high temperature durability of the components within the hot gas path of the engine must correspondingly increase. Significant advances in high temperature capabilities have been achieved through the formulation of nickel and cobalt-base superalloys. Nonetheless, when used to form components of the turbine, combustor and augmentor sections of a gas turbine engine, such alloys alone are often susceptible to damage by oxidation and hot corrosion attack and may not retain adequate mechanical properties. For this reason, these components are often protected by a thermal barrier coating (TBC) system. TBC systems typically include an environmentally-protective bond coat and a thermal-insulating ceramic topcoat, often referred to as the TBC. Bond coat materials widely used in TBC systems include oxidation-resistant overlay coatings such as MCrAlX (where M is iron, cobalt and/or nickel, and X is yttrium, another rare earth element, or a reactive element such as zirconium), and oxidation-resistant diffusion coatings such as diffusion aluminides that contain aluminum intermetallics.
Zirconia (ZrO2) that is partially or fully stabilized by yttria (Y2O3), magnesia (MgO) or another alkaline-earth metal oxide, ceria (CeO2) or another rare-earth metal oxide, or mixtures of these oxides has been employed as TBC materials. Binary yttria-stabilized zirconia (YSZ) has particularly found wide use as the TBC material on gas turbine engine components because of its low thermal conductivity, high temperature capability including desirable thermal cycle fatigue properties, and relative ease of deposition by plasma spraying, flame spraying and physical vapor deposition (PVD) techniques such as electron beam physical vapor deposition (EBPVD). TBC""s employed in the highest temperature regions of gas turbine engines are often deposited by PVD, particularly EBPVD, which yields a strain-tolerant columnar grain structure that is able to expand and contract without causing damaging stresses that lead to spallation. Similar columnar microstructures can be produced using other atomic and molecular vapor processes, such as sputtering (e.g., high and low pressure, standard or collimated plume), ion plasma deposition, and all forms of melting and evaporation deposition processes (e.g., cathodic arc, laser melting, etc.). In contrast, plasma spraying techniques such as air plasma spraying (APS) deposit TBC material in the form of molten xe2x80x9csplats,xe2x80x9d resulting in a TBC characterized by a degree of inhomogeneity and porosity.
As is known in the art, zirconia is stabilized with the above-noted oxides to inhibit a tetragonal to monoclinic phase transformation at about 1000xc2x0 C., which results in a volume expansion that can cause spallation. At room temperature, the more stable tetragonal phase is obtained and the undesirable monoclinic phase is minimized if zirconia is stabilized by at least about six weight percent yttria. An yttria content of seventeen weight percent or more ensures a fully stable cubic phase. Though thermal conductivity of YSZ decreases with increasing yttria content, the conventional practice has been to stabilize zirconia with at least six weight percent, and more typically to only partially stabilize zirconia with six to eight weight percent yttria (6-8% YSZ) with the understanding that 6-8% YSZ TBC is more adherent and spall-resistant to high temperature thermal cycling than YSZ TBC containing greater and lesser amounts of yttria. Limited exceptions have generally included plasma-sprayed zirconia said to be stabilized by mixtures of yttria, magnesia, calcia or ceria, to which certain oxides may be added at specified levels to obtain a desired effect. For example, according to U.S. Pat. No. 4,774,150 to Amano et al., Bi2O3, TiO2, Tb4O7, Eu2O3 and/or Sm2O3 may be added to certain layers of a TBC formed of zirconia stabilized by yttria, magnesia or calcia, for the purpose of serving as xe2x80x9cluminous activators,xe2x80x9d and U.S. Pat. No. 4,996,117 to Chu et al. discloses forming a TBC (e.g., zirconia stabilized with yttria or magnesia) whose individual particles are coated with a corrosion-resistant layer of silica (SiO2), alumina (Al2O3), an aluminum silicate, a zirconium silicate, an aluminum titanate, or a mixture thereof.
Contrary to the conventional practice of stabilizing zirconia with at least six weight percent yttria, U.S. Pat. No. 5,981,088 to Bruce showed that zirconia partially stabilized by less than six weight percent yttria exhibits superior erosion and impact resistance as compared to conventional YSZ. The basis for this improvement is not well understood, though it is believed that YSZ TBC containing less than six weight percent yttria, particularly about four weight percent yttria, exhibits increased fracture toughness that is responsible for improved erosion and impact resistance.
In order for TBC to remain effective throughout the planned life cycle of the component it protects, it is important that the TBC maintains a low thermal conductivity throughout the life of the component. However, the thermal conductivity of columnar YSZ TBC is known to increase over time when subjected to the operating environment of a gas turbine engine as a result of grain and/or pore coarsening or redistribution. Consequently, YSZ TBC is often deposited on gas turbine engine components to a greater thickness than would otherwise be necessary. Alternatively, internally cooled components such as blades and nozzles must be designed to have higher cooling flow. Both of these solutions are undesirable for reasons relating to cost, component life and engine efficiency.
In view of the above, it can be appreciated that further improvements in TBC technology are desirable, particularly as TBC""s are employed to thermally insulate components intended for more demanding engine designs.
The present invention generally provides a coating material, such as for a thermal barrier coating (TBC) of a component intended for use in a hostile environment, such as the superalloy turbine, combustor and augmentor components of a gas turbine engine. The coating material is zirconia that is partially stabilized with yttria (YSZ), preferably not more than three weight percent yttria, and to which one or more additional metal oxides are alloyed to increase crystallographic defects and lattice strain energy in the coating grains and, optionally, form second phases of zirconia and/or compound(s) of zirconia and/or yttria and the additional metal oxide(s). According to the invention, increasing the crystallographic defects and lattice strain energy within the YSZ lattice significantly reduces the thermal conductivity of the YSZ compared to that obtained with conventional 6-8% YSZ. Improvements obtained by this invention are particularly evident with YSZ coatings having a columnar grain structure, such as those deposited by EBPVD and other PVD techniques, though the invention is also applicable to coatings deposited by such methods as plasma spraying.
In the present invention, increased crystallographic defects and lattice strain energy are the result of composition-induced defect reactions in a coating that consists essentially of zirconia partially stabilized by up to three weight percent yttria, and to which is alloyed very specific amounts of one or more additional metal oxides that have solid solubility in zirconia and are responsible for the desired defect reactions and/or increased lattice strain due to ion size differences. These metal oxides are limited to the alkaline-earth metal oxides magnesia (MgO), calcia (CaO), strontia (SrO) and barium oxide (BaO), the rare-earth metal oxides lanthana (La2O3), ceria (CeO2), neodymia (Nd2O3), gadolinium oxide (Gd2O3) and dysprosia (DY2O3), as well as such metal oxides as nickel oxide (NiO), ferric oxide (Fe2O3), cobaltous oxide (CoO), and scandium oxide (Sc2O3). If present in an appropriate and limited amount, each of the above metal oxides has the effect of increasing crystallographic defects and/or lattice strains in the coating grains by affecting the presence of metal or oxygen sublattice vacancies, and/or causing strains from the substitution of different size metal atoms on zirconium sites. Notably, the degree to which crystallographic defects and/or lattice strain is required to be increased with the additional metal oxides of this invention excludes such oxides as hafnia (HfO2), titania (TiO2), tantalum oxide (Ta2O5), niobium oxide (Nb2O5), erbia (Er2O3) and ytterbia (Yb2O3), as well as others.
According to the invention, increasing the number of defects and/or strain energy in the YSZ lattice by the inclusion of the above-noted oxides serves to significantly increase the resistance to heat transfer through YSZ, and therefore through a coating (e.g., TBC) formed of the YSZ. These oxides also have the capability of promoting the formation of precipitates of zirconia and/or compound(s) of zirconia and/or yttria and the additional metal oxide(s). These second phase precipitates are believed to provide scattering sites for lattice vibrations (phonons), which contribute to the thermal conductivity of the coating.
As a result of exhibiting greater resistance to heat transfer, YSZ TBC""s in accordance with this invention can be subsequently heated to temperatures encountered within the hot gas path of a gas turbine engine and, though grain and pore coarsening may occur, the TBC will maintain a thermal conductivity at a level equal to or lower than that possible with conventional 6-8% YSZ TBC subjected to identical conditions. Therefore, gas turbine engine components can be designed for thinner TBC and/or, where applicable, lower cooling air flow rates, which reduces processing and material costs and promotes component life and engine efficiency.
Other objects and advantages of this invention will be better appreciated from the following detailed description.